Multidecadal trends in ultraviolet radiation, temperature, and dissolved oxygen have altered vertical habitat availability for Daphnia in temperate Lake Giles, USA

Long-term browning has resulted in increases in dissolved organic carbon and reduced water clarity that have altered the vertical physical structure of many lake ecosystems. The primary responses include reduced ultraviolet (UV) penetration, warming surface waters, and decreased deepwater dissolved oxygen concentrations that interactively alter vertical habitat suitability for zooplankton. Over 3 decades, Daphnia populations have decreased in abundance and shallowed in their vertical distribution in temperate Lake Giles (Pennsylvania, USA). Using 3 decades of corresponding long-term vertical profile data of UV radiation, water temperature, and dissolved oxygen, we modelled both the suitable vertical habitat and thermally optimal habitat for Daphnia to understand the potential role of habitat availability for their population dynamics. The vertical extent of suitable habitat increased over time primarily due to strong decreases in UV penetration. In contrast, thermally optimal habitat decreased due to strong increases in vertical thermal gradients that were especially strong in late summer. The vertical distribution of Daphnia became shallower over this time period and may be a response to lower UV exposure near the surface, but continued warming of surface waters and decreasing deepwater dissolved oxygen concentrations are likely to lead to a vertical habitat squeeze. The biological implications of long-term browning require more attention due to the complex and important implications for population dynamics, species interactions, and food web structure in lakes.